The present invention is generally directed to processes for the preparation of toner resins and toners. More specifically, the present invention relates to melt mixing processes, batch or continuous, but preferably continuous processes such as, for example, reactive extrusion for preparing crosslinked toner resins. The present invention also relates to processes for crosslinking reactive linear resins for the preparation of crosslinked toner resins that can be selected for application in heat fixable toners with superior fusing and vinyl offset performance.
Toner utilized in development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, numerous polymers are known, including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black, magnetite and various colored pigments may be selected, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, metallic alkyl salicylates, and the like are known.
To fix the toner to a support medium, such as a sheet of paper or transparency, hot roll fixing is commonly used. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium, forming a fixed image. Such a fixing system is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is called the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is called the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred or extraneous images. This undesirable phenomenon is called offsetting. Between the COT and HOT of the toner, is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is called the Fusing Latitude.
The hot roll fixing system and a number of toners used therein, however, exhibit several problems. First, the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Second, offsetting phenomena can be a problem. Third, toner containing vinyl type binder resins such as styrene-acrylic resins may have an additional problem known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with, for example, a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl notebook binder covers, and the fixed image adheres to the PVC surface.
Thus, there remains a need for toner resins with low fix temperature and high offset temperature and broad fusing latitude, superior or nonexistent vinyl offset property, and efficient and economic processes for the preparation of such resins.
In order to prepare lower fix temperature resins for toner, the molecular weight of the resin may be lowered. Low molecular weight and amorphous polyester resins and epoxy resins have been used to prepare low temperature fixing toners. For example, attempts to produce toners utilizing polyester resins as binder are disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be rendered lower than that of other materials, such as styrene-acrylic resins. However, this may lead to a lowering of the hot offset temperature and, as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent fuser roll offsetting and to increase fusing latitude of toners, modification of the binder resin structure by conventional polymerization processes, for example, by branching, crosslinking, and the like, has been attempted. For example, in U.S. Pat. No. 3,681,106 to Burns et al., a process is disclosed whereby a polyester resin was improved with respect to offset resistance by non-linearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is by crosslinking during polymerization. In U.S. Pat. No. 3,941,898 to Sadamatsu et al., for example, a crosslinked vinyl type polymer prepared using conventional crosslinking was used as the binder resin. Similar disclosures for vinyl type resins are presented in U.S. Pat. No. Re. 31,072 (a reissue of U.S. Pat. No. 3,938,992) to Jadwin et al., U.S. Pat. No. 4,556,624 to Gruber et al., U.S. Pat. No. 4,604,338 to Gruber et al., and U.S. Pat. No. 4,824,750 to Mahalek et al. Also, disclosures have been made of crosslinked polyester binder resins using conventional polycondensation processes for improving offset resistance, such as for example in U.S. Pat. No. 3,681,106 to Burns et al.
While significant improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue with crosslinked resins prepared by conventional polymerization, for example, vinyl type processes including solution, bulk, suspension and emulsion polymerizations and polycondensation processes. In all of these processes, monomer and crosslinking agent are added to the reactor at the same time. The crosslinking reaction is not very fast and chains can grow in more than two directions at the crosslinking point by the addition of monomers. Three types of polymer configurations are produced--a linear and soluble portion called the linear portion, a crosslinked portion which is low in crosslinking density and therefore is soluble in some solvents, such as, tetrahydrofuran, toluene, and the like, and is called sol, and a portion comprising highly crosslinked gel particles which is not substantially soluble in any solvent, for example, tetrahydrofuran, toluene and the like, and is called gel. The second portion with low crosslinking density (sol) is responsible for widening the molecular weight distribution of the soluble part which results in an elevation of the minimum fixing temperature of the toner. Also, a drawback of these processes, which are not carried out under high shear, is that as more crosslinking agent is used the gel particles or very highly crosslinked insoluble polymer with high molecular weight increase in size. The large gels can be more difficult to disperse pigment in, causing unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, in the case of vinyl polymers, the toners produced often show vinyl offset.
In U.S. Pat. No. 4,533,614 to Fukumoto et al., a process was utilized for preparing loosened crosslinked polyester binder resin which showed low temperature fix and good offset resistance. Metal compounds were used as crosslinking agents. Similar disclosures are presented in U.S. Pat. No. 3,681,106 to Burns et al. and Japanese Laid-open Patent Applications Nos. 94362/1981, 116041/1981 and 166651/1980. As discussed in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, in the case of color toners other than black, for example cyan, metal complexes can adversely affect the color of the pigments. It is also known that metal containing toner can have disposal problems in some areas, such as in the State of California, U.S.A. Metal complexes are often also expensive materials.
Reactive extrusion processes for producing engineering plastics are known, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modifying reactions, such as graft, coupling and degradation reactions. However, it is believed that the prior art does not disclose the use of a reactive extrusion process to prepare crosslinked thermoplastic resins for use in toners.
In U.S. Pat. No. 4,894,308 to Mahabadi et al. and U.S. Pat. No. 4,973,439 to Chang et al., for example, extrusion processes are disclosed for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into the binder resin in the extruder. However, in each of these patents, there is no suggestion of a chemical reaction occurring.
An injection molding process for producing crosslinked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736 to Takiura in which polyolefin or polyvinyl chloride resin and crosslinking agent was mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the crosslinking reaction occurred at increased temperature and pressure, and at low or zero shear.
In U.S. Pat. No. 4,089,917 to Takiura et al., an injection molding and crosslinking process is disclosed in which polyethylene resin and crosslinking agent were mixed in an extruder and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. However, the crosslinking reaction still took place in the reaction chambers at low or zero shear, and the final product is a thermoset molded part, and thus, is not useful for thermoplastic toner resins.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293 to Macosko et al. in which polyurethane precursor systems were crosslinked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress was greater than a critical value in order to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, and thus, is not useful for thermoplastic toner resins.
The processes disclosed in U.S. Pat. Nos. 3,876,736, 4,089,917, and 4,990,293 are not considered reactive extrusion processes, since, for example, the crosslinking occurs in a die or a mold, and not in an extruder. These processes are for producing engineering plastics such as thermoset materials which cannot be remelted once molded, and thus are not suitable for toner application.